In a horse-race matchup of likely voters, King led with 55 percent of those polled. Republican Charlie Summers took 27 percent, while Democrat Cynthia Dill took 7 percent. Nine percent didn't know for whom they would vote.

King is widely expected to caucus with Democrats if he wins, though he has not publicly announced which party he would caucus with, if any.

The poll, commissioned by the Portland Press Herald and conducted by the nonpartisan firm Critical Insights, was not released in full by either entity.

But in the Press Herald write-up of its results, King's broad support was noted.

"King's appeal crosses a wide spectrum, according to the poll, with more than 50 percent support among almost every subgroup of voters: men, women, young, old, less educated, more educated, low-income, high-income," the Press Herald wrote. "He trails only among Republicans and self-described conservatives, who favor Summers."

The poll also found President Barack Obama leading presumptive GOP presidential candidate Mitt Romney in the state, 49 percent to 35 percent.

The survey used random-digit dialing to call 615 likely voters on land lines and cellphones June 20-25. The margin of error was 4 points.